This invention relates to electronic signal processing apparatus employing a plurality of signal delaying stages of the charge transfer device (CTD) type.
A charge coupled device (CCD), which is one version of the charge transfer device, is well suited for processing analog signals. These devices include input structures for converting a signal voltage or current to an amount or packet of charge in a transfer channel, and output structures for measuring the charge in the packet at the output of the channel.
A considerable amount of design effort has been expended to provide substantially linear conversion from input signal voltage or current to charge and back again. Any non-linearities in these processes are particularly troublesome where a plurality of signals are supplied to a plurality of inputs of a CCD for combining in predetermined proportions to produce a desired filter characteristic. Such a situation is encountered, for example, where the CCD is a part of a comb filter for color television signals and is constructed in the manner described in U.S. pat. application Ser. No. 781,303 entitled "Electronic Signal Processing Apparatus" filed Mar. 25, 1977, in the name of D. H. Pritchard and assigned to the same assignee as the present invention, now U.S. Pat. No. 4,096,516.
In the Pritchard comb filter, a composite video signal, including frequency interleaved luminance and chrominance signal components, is supplied to a first signal path including a number of delay elements arranged to exhibit a total delay incrementally greater than the period of a television line scanning interval (i.e., 1H). The composite video signal is also supplied to second and third signal paths, each of which is arranged to exhibit a delay equal to the incremental difference between the line scanning interval and the total delay of the first path. The differences in delay between the first and second paths, and between the first and third paths, are thereby equal to 1H. The delay is accurately determined since it is dependent substantially only upon the difference in the number of delay stages (a geometric certainty) and upon the clock frequency associated with the signal transfer. The clock frequency can be controlled to a high degree of accuracy, for example, by use of a crystal oscillator. Luminance signals are derived by additively combining the signals from the first and second paths while chrominance signals are derived by subtractively combining the signals from the first and third paths. The subtraction is most readily performed by inverting the composite video signal supplied to the third path and then adding the outputs of the first and third paths. This latter addition of signals provides a comb filter response with maxima at the color subcarrier frequency and all other odd multiples of one-half the line scanning frequency, while the addition of signals from the first and second paths provides a comb filter response with maxima at multiples of the line scanning frequency. The relative depth of the notches in each of the two filter responses depends upon the accuracy with which the amplitude and phase responses for the two associated signal paths are matched and the accuracy of the difference in time delay between the two paths. The accuracy of the time delay may be set consistently by the delay difference technique of Pritchard.
The present invention provides practical solutions to the problems of matching amplitude and phase responses of two or more signal delay systems, providing proper weighting of the signals from two such systems to achieve a desired degree of signal cancellation, and reproducing characteristics from one CCD circuit chip to another and providing a reasonable economy of chip area.